1. Field of the Invention
This invention relates to an induction heating apparatus which is effective when used as a heating and fixing device in an image forming apparatus such as a copier or a printer.
2. Related Background Art
The present invention relates, for example, to an induction heating apparatus (a heating apparatus of an electromagnetic induction heating type) used as a fixing device or the like for heating and fixing a heat-meltable powder image (visualizing agent image) such as a toner image on paper in an image forming apparatus such as a copier.
Particularly, it relates to an induction heating apparatus for supplying electric power to a plane type heating coil (induction heating coil) and induction-heating an object to be heated.
It also relates to an image forming apparatus provided with the induction heating apparatus as image heating means.
There have been devised various methods of induction-heating a planar object and various heating coil shapes. Among these, a planar type heating coil shape exhibits relatively high heating efficiency because the area of the heating coil which faces a planar object to be heated can be secured widely.
FIGS. 6A, 6B, 7A and 7B of the accompanying drawings show an example of such an induction heating apparatus. This example of the apparatus is an image heating and fixing device of an induction heating type and a film heating type, and FIGS. 6A and 6B are a transverse side model view and a longitudinal front model view, respectively, of the essential portions of the apparatus, and FIG. 7A is a plan model view of a plane type and ellipse-like heating coil.
In FIGS. 6A, 6B, 7A and 7B, the reference numeral 11 designates an induction heat generating member comprising a magnetic member as an object to be heated (heater or heating member), and for example, a sideways long, flat, thin iron plate. The reference numeral 12 denotes a heat-resisting holding member fixedly holding the object 11 to be heated, the reference numeral 13 designates heat-resisting thin film (hereinafter referred to as the fixing film), and the reference numeral 14 denotes an elastic pressing roller.
The elastic pressing roller 14 is formed with a heating nip portion (fixing nip portion) N of a predetermined nip width and is brought into pressure contact with the underside of the object 11 to be heated fixedly held by the holding member 12 against the elasticity thereof with the fixing film 13 interposed therebetween.
The reference numeral 1 designates a plane type and ellipse-like heating coil fixedly held and disposed on the upper surface side of the object to be heated which is the opposite side to the fixing nip portion N side of the object 11 to be heated fixedly held by the holding member 12, in face-to-face relationship with the object 11 to be heated. The length and width of this plane type and ellipse-like heating coil 1 are substantially the same as the length and width of the object 11 to be heated.
A high frequency current (high frequency AC electric power or an induction current for heating) is supplied from an exciting circuit, not shown, to the heating coil 1, whereby an AC magnetic field (alternating magnetic flux) is produced, and the AC magnetic field acts on the object 11 to be heated which is an induction heat generating member comprising a magnetic member, whereby an induction current is induced in the object 11 to be heated, and the object 11 to be heated generates heat (Joule heat by eddy current loss) by induction heating and rises in temperature.
The temperature rise of the object 11 to be heated is detected by a temperature measuring element (temperature detecting element), not shown, and the supply of electric power from the exciting circuit to the heating coil 1 is controlled by a temperature control system, not shown, and the temperature control of the object 11 to be heated is done so that the measured temperature may be maintained at a predetermined fixing temperature.
The fixing film 13 is a cylindrically shaped or endless belt-shaped or rolled end-having web-shaped member, and is conveyed in the direction of arrow by driving means, not shown, or the rotatively driving force of the pressing roller 14 while sliding in close contact with the underside of the object 11 to be heated in the fixing nip portion N.
When in a state in which the object 11 to be heated has been induction-heated to a predetermined temperature and the fixing film 13 has been conveyed in the direction of arrow, a recording material P bearing an unfixed toner image t formed thereon is introduced into between the fixing film 13 and the pressing roller 14 in the fixing nip portion N, the recording material P is conveyed through the fixing nip portion N with the fixing film 13 while being in close contact with the surface of the fixing film 13.
In this fixing nip portion N, the recording material P and the toner image t thereon are heated by the object 11 to be heated through the fixing film 13 and the toner image t on the recording material P is fixed.
The recording material portion which has passed through the fixing nip portion N curvature-peels from the surface of the fixing film 13 and is conveyed.
As previously described, the shape of the plane type heating coil enables the area of the heating coil 1 which faces the plane-like object 11 to be heated to be secured widely and therefore exhibits relatively high heating efficiency, and by using the coil shape as described above, that portion of the object to be heated which corresponds to the lengthwisely central portion of the heating coil 1 becomes relatively uniform in heat generation distribution, but the longitudinally opposite end portions of the heating coil 1 do not become uniform in the distribution of a magnetic field and therefore, those portions of the object to be heated which correspond to the longitudinally opposite end portions of the heating coil 1, i.e., the longitudinally opposite end portions of the object to be heated, cause abnormal heat generation, and correspond to the opposite end portions of the heating portion and therefore, it has often been the case that as shown in FIG. 7B of the accompanying drawings, those portions become low temperature portions from which the heat escapes to the outside.